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Apache XML Security for Java supports XML-Signature Syntax and Processing, W3C Recommendation 12 February 2002, and XML Encryption Syntax and Processing, W3C Recommendation 10 December 2002. As of version 1.4, the library supports the standard Java API JSR-105: XML Digital Signature APIs.
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/**
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements. See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership. The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied. See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */
package org.apache.xml.security.c14n.implementations;

import java.io.IOException;
import java.io.OutputStream;
import java.util.Map;
import java.util.Set;
import java.util.SortedSet;
import java.util.TreeSet;

import org.apache.xml.security.c14n.CanonicalizationException;
import org.apache.xml.security.c14n.Canonicalizer;
import org.apache.xml.security.parser.XMLParserException;
import org.apache.xml.security.signature.XMLSignatureInput;
import org.w3c.dom.Attr;
import org.w3c.dom.Comment;
import org.w3c.dom.DOMException;
import org.w3c.dom.Element;
import org.w3c.dom.NamedNodeMap;
import org.w3c.dom.Node;
import org.w3c.dom.ProcessingInstruction;

/**
 * Serializes the physical representation of the subtree. All the attributes
 * present in the subtree are emitted. The attributes are sorted within an element,
 * with the namespace declarations appearing before the regular attributes.
 * This algorithm is not a true canonicalization since equivalent subtrees
 * may produce different output. It is therefore unsuitable for digital signatures.
 * This same property makes it ideal for XML Encryption Syntax and Processing,
 * because the decrypted XML content will share the same physical representation
 * as the original XML content that was encrypted.
 */
public class CanonicalizerPhysical extends CanonicalizerBase {

    /**
     * Constructor Canonicalizer20010315
     */
    public CanonicalizerPhysical() {
        super(true);
    }

    /**
     * Always throws a CanonicalizationException.
     *
     * @param xpathNodeSet
     * @param inclusiveNamespaces
     * @param writer OutputStream to write the canonicalization result
     * @throws CanonicalizationException always
     */
    @Override
    public void engineCanonicalizeXPathNodeSet(Set xpathNodeSet, String inclusiveNamespaces, OutputStream writer)
        throws CanonicalizationException {

        /** $todo$ well, should we throw UnsupportedOperationException ? */
        throw new CanonicalizationException("c14n.Canonicalizer.UnsupportedOperation");
    }

    /**
     * Always throws a CanonicalizationException.
     *
     * @param rootNode
     * @param inclusiveNamespaces
     * @param writer OutputStream to write the canonicalization result
     * @throws CanonicalizationException
     */
    @Override
    public void engineCanonicalizeSubTree(Node rootNode, String inclusiveNamespaces, OutputStream writer)
        throws CanonicalizationException {

        /** $todo$ well, should we throw UnsupportedOperationException ? */
        throw new CanonicalizationException("c14n.Canonicalizer.UnsupportedOperation");
    }

    /**
     * Always throws a CanonicalizationException.
     *
     * @param rootNode
     * @param inclusiveNamespaces
     * @param writer OutputStream to write the canonicalization result
     * @throws CanonicalizationException
     */
    @Override
    public void engineCanonicalizeSubTree(
            Node rootNode, String inclusiveNamespaces, boolean propagateDefaultNamespace, OutputStream writer)
            throws CanonicalizationException {

        /** $todo$ well, should we throw UnsupportedOperationException ? */
        throw new CanonicalizationException("c14n.Canonicalizer.UnsupportedOperation");
    }

    /**
     * Output the Attr[]s for the given element.
     * 

     * The code of this method is a copy of
     * {@link #outputAttributes(Element, NameSpaceSymbTable, Map, OutputStream)},
     * whereas it takes into account that subtree-c14n is -- well -- subtree-based.
     * So if the element in question isRoot of c14n, it's parent is not in the
     * node set, as well as all other ancestors.
     *
     * @param element
     * @param ns
     * @param cache
     * @param writer OutputStream to write the canonicalization result
     * @throws CanonicalizationException, DOMException, IOException
     */
    @Override
    protected void outputAttributesSubtree(Element element, NameSpaceSymbTable ns,
                                           Map cache, OutputStream writer)
        throws CanonicalizationException, DOMException, IOException {
        if (element.hasAttributes()) {
            // result will contain all the attrs declared directly on that element
            SortedSet result = new TreeSet<>(COMPARE);

            NamedNodeMap attrs = element.getAttributes();
            int attrsLength = attrs.getLength();

            for (int i = 0; i < attrsLength; i++) {
                Attr attribute = (Attr) attrs.item(i);
                result.add(attribute);
            }

            //we output all Attrs which are available
            for (Attr attr : result) {
                outputAttrToWriter(attr.getNodeName(), attr.getNodeValue(), writer, cache);
            }
        }
    }

    @Override
    protected void outputAttributes(Element element, NameSpaceSymbTable ns,
                                    Map cache, OutputStream writer)
        throws CanonicalizationException, DOMException, IOException {

        /** $todo$ well, should we throw UnsupportedOperationException ? */
        throw new CanonicalizationException("c14n.Canonicalizer.UnsupportedOperation");
    }

    @Override
    protected void circumventBugIfNeeded(XMLSignatureInput input)
        throws XMLParserException, IOException {
        // nothing to do
    }

    @Override
    protected void handleParent(Element e, NameSpaceSymbTable ns) {
        // nothing to do
    }

    /** {@inheritDoc} */
    @Override
    public final String engineGetURI() {
        return Canonicalizer.ALGO_ID_C14N_PHYSICAL;
    }

    @Override
    protected void outputPItoWriter(ProcessingInstruction currentPI,
                                    OutputStream writer, int position) throws IOException {
        // Processing Instructions before or after the document element are not treated specially
        super.outputPItoWriter(currentPI, writer, NODE_NOT_BEFORE_OR_AFTER_DOCUMENT_ELEMENT);
    }

    @Override
    protected void outputCommentToWriter(Comment currentComment,
                                         OutputStream writer, int position) throws IOException {
        // Comments before or after the document element are not treated specially
        super.outputCommentToWriter(currentComment, writer, NODE_NOT_BEFORE_OR_AFTER_DOCUMENT_ELEMENT);
    }

}